Process for improving gloss retention and chalking resistance of rutile pigments



United States Patent 6 Claims. 61. 106-300) The present invention relates in general to TiO pigment having improved. pigmentary properties and more particularly to a superior method for producing a TiO pigment having improved gloss retention and chalking resistance.

The paint and varnish industries are continually increasing their stringent specifications in the fields of paint, air-drying varnishes and baking enamels with respect to the chalking resistance and gloss retention of the TiO pigment used in its products; and attempts have been made for quite some time to improve the chalking resistance and gloss retention of the TiO pigment. For example the calcined and milled TiO pigment has been .slurried in water, with the addition of a dispersing agent, as the case may be, followed by wet milling and/or classification after which it has been mixed with a water-soluble silicate and/or a water-soluble aluminum salt and/or other metal salts and alkali, filtered, Washed, dried and milled. Such post-calcination treated TiO pigments show some improvement in chalking resistance as compared with that of untreated TiO pigment. However, still better chalking resistance is desirable in some fields of application. In addition, an attempt has been made to improve the pigmentary properties of a post-calcination treated pigment by subsequent calcination. However TiO pigments so prepared still fail to have the high gloss retention and chalking resistance presently required by the industry.

An object, therefore, of the present invention is to provide a relatively simple and economical method for producing an improved Ti0 pigment for use in paints, var nishes and enamels. A further object of the invention is to provide an improved method for producing a rutile -TiO pigment which will retain its high gloss and resist chalking even after long periods of weathering. These and other objects, features and advantages of the invention will be described and illustrated more fully in the description which follows hereinafiter.

In its broadest aspects the present invention relates to an improved method for producing a Ti0 pigment having superior gloss retention and chalking resistance wherein a calcined TiO pigment is treated initially with watersoluble salts of aluminum and silicon to coat the pigment with the corresponding hydrous oxides after which the oxide coated pigment is treated a second time with Watersoluble salts of aluminum and silicon, or titanium, aluminum and silicon, to form a second coating of the corresponding hydrous oxides on the pigment.

The TiO pigment used in the process of the instant invention may be either an anatase or rutile modification but is preferably of the rutile modification and may be the product of the so-called sulfate process wherein the pigment is produced from a hydrate precipitated from a titanium sulfate solution; or the product of the so-called vapor phase process wherein vaporous TiCl, is reacted with oxygen at elevated temperatures to form TiO t The process of the invention is characterized by two successive post-calcination treatments, the first being carried out by wet-milling an aqueous slurry of calcined TiO with the aid of a dispersing agent, if necessary, followed by classification; and then adding to the classi- 3,251,705 Patented May 17, 1966 ice fied pigment, in any desired sequence, and with thorough mixing, a water-soluble silicate as for example sodium silicate, a water-soluble aluminum salt such as for example aluminum sulfate and/ or, if desired other soluble metal compounds or salts capable of forming insoluble substantially colorless compounds; or substantially insoluble hydrous metal oxides. In the event the suspension so formed is acid an alkali or ammonia is added to neutralize the acid suspension and additional anions are added to precipitate the metal compounds as insoluble hydrous metal oxides on the pigment. Should the suspension be alkaline it is neutralized by the addition of an acid.

Following precipitation of the insoluble metal oxides as ahydrous metal oxide coating on the pigment, the suspension is neutralized by the addition of an alkali or ammonia and filtered after which the coated pigment is washed, dried and milled.

The second post-calcination treatment has been found not only to improve gloss retention and chalking resistance of the pigment but also other properties such as brightness, color-tone and dispersibility and is carried out in a manner similar to the first post-calcination treatment but using as the pigment source, the hydrous metal oxide coated pigment produced by the first post-calcination treatment. Moreover, while in the first post-calcination treat-. ment it is essential that the pigment slurry be wet-milled and/or classified frequently in order to maintain a good dlspersion of the relatively coarse aggregates previously produced during calcination of the pigment, no wet-milling and/or classification is necessary during the second.

ing'results have been achieved where a water-soluble ti tamum salt is used in conjunction with the water-soluble salts of silicon and aluminum in the secondpost-calcination treatment.

namely, titanium-silicate-aluminum.

' The amount of soluble metal salts used are generally in the range of from 0.5 to 5.0% calculated as the oxides; on a TiO weight-basis, the specific amounts employed depending upon the type of soluble salt used.

Particularly good results are obtained'according to the process of this invention if, in the first post-calcination treatment, the silica and aluminum salts are used in amounts ranging from 1% t0 2% calculated as the oxides on a TiO weight basis. The use of less than 0.5% calculated as the oxide, of each metal salt in the first post-calcination treatment gives unsatisfactory results and hence is not suitable while amounts in excess of 5.0% etfect no appreciable improvements over the results achieved by using 5.0% and hence are uneconomical. In general the preferred range is from 1.02.0% calculated as the oxides on a TiO basis but it will be understood that other proportions may be used within the range of from (LS-5.0%.

While some improvement in gloss retention and chalking resistance has been achieved by using an aluminum salt alone the results are not as good as when the aluminum salt is used in conjunction with the silicate; while the use of the silicate alone is prone to create filtering problems and produce a pigment having inferior pigmentary properties.

As pointed out above the treatment steps employed in the first and second post-calcination treatments are quite similar and accordingly the amounts of the water- Moreover, with this combination of water-soluble salts it has been found especially advan-' tageous to add the aforesaid salts in a particular sequence,

soluble salts used in the second post-calcination treatment may be and generally are in the same range as those used in the first post-calcination treatment.

Further as to the amounts of water-soluble salts used, it has been found that the improvements in gloss retention and chalking resistance achieved by the present invention are attributed only slightly to an increase in the amount of hydrous metal oxide on the pigment, i.e., to coat the pigment in one treatment step withhydrous oxides in an amount equivalent to that deposited in two successive steps will not produce a pigment hav ing as good pigmentary characteristics as those of a pigment which has been subjected to two successive treatments. In brief the remarkable improvements in gloss retentionand chalking resistance effected by the invention are due primarily to treating the pigment with the water-soluble salts of silica, aluminum and/or titanium in two successive post-calcination treatment steps and not to the total amount of hydrous metal oxides deposited on the pigment.

In order to test the hydrous metal oxide coated TiO pigment produced according to the process of this invention for gloss retention and chalking resistance the coated pigment was incorporated in a varnish prepared as follows: A dispersion was prepared of a 50% solution of a long-oil linseed oil, an alkyd resin, e.g., Alkydal L Extra, and sutficient hydrous metal oxide coated rutile TiO pigment, prepared according to the instant invention, to provide a pigment to solid vehicle ratio of 0.8:1. To this dispersion was added a drier which in this instance was a cobalt-lead naphthenate. The mixture'was then sent twice through a three-roll roller mill to form a uniform dispersion of the pigment in the alkyd resin vehicle.

The varnish so formed .was then applied by means of a conventional film-forming device to a glass plate to form a layer thereon 100p. in thickness. After drying the layer of varnish for 48 hours at room temperature a second layer of varnish was applied in a manner similar to that of the first after which the varnished plates were stored for 8 days.

The varnished plates were then taken from storage and weathered for from 380 to 520 hours (see examples for specific periods) in a standard Xenotest W device of the Qnarzlampen G.m.b.H., Hanan, using regular procedures wherein the air humidity stood at about 80%. The gloss value of each weathered varnished plate was determined by means of the 20 glossmeter test using" a Gardener 20 Glossmeter (Gardener Laboratory, Bethesda, Maryland). Chalking was determined by means of a Kempf stamper (Erichsen Co., HemerSundwig, Westphalia, Germany) using a rubber stamper 2 cm. in diameter. The degree of chalking was estimated visually.

The following examples serve to illustrate the invention in greater detail. The product of each example was tested for gloss retention and chalk resistance according to the methods described above and the results are recorded below in Tables 1 and 2.

Example 1 Calcined rutile TiO pigment was slurried in de'mineralized water with the addition of sodium hexametaphosphate as dispersing aid. The suspension was freed of all coarse particles by wet-milling in a ball mill and classifying in a centrifuge. treatment one liter of this suspension (corresponding to 300 g. Ti was heated to 60 C. The following additions were made consecutively to the slurry while stirring continuously and maintaining the temperature constant:

(1) 28.4 ml. of a sodium silicate solution containing 190 g.p.l. SiO corresponding to 1.8% SiO on the basis of the pigment used; after addition of the silicate stirring was continued for minutes.

For the first post-calcination (2) A solution of 41.2 g. Al (SO -18H O in 100 ml. water, corresponding to2.1% A1 0 on the basis of pagment used; following the addition of the aluminum salt stirring was continued for 10 minutes.

(3) Dilute ammonia solution up to a pH 8.1; stirring was continued for 'minutes after addition of the ammonia solution.

The pH of the suspension was measured repeatedly and maintained at about 8.1 by further addition, if necessary, of dilute ammonia. water by suction, washed repeatedly with demineralized water and dried in an electric drying oven at 120C. for 15-20 hours. Subsequently the pigment was finally milled. 7

After the first post-calcination treatment the oxidecoated rutile pigment was subjected to a second postcalcination treatment as follows: 300 g. of the oxidecoated pigment were slurried in demineralized water with the addition of sodium hexametaphosphate, as a dispersing aid, and soda lye; the volume of the suspension was 1 liter, the pH value 9.5-10.0. The suspension was then heated to 60 C. The followingadditions were made to the slurry while stirring vigorously:

( 1) 31.6 ml. of a water glass solution with a content of 190 g.p.l. SiO corresponding to 2.0% SiO on the basis of pigment used; stirring was continued for 10 minutes.

(2) A solution of 39.2 g. Al '(SO -1-8 H O in ml. water, correspondingto 20% A1 0 on the basis of pigment used; stirring was continued for 10 minutes after addition of the aluminum salt.

(3) Dilute ammonia solution up to pH 8.1; stirring was continued for 30 minutes and the pH was readjusted to 8.1.

The suspension was then dewatered and washed repeatedly with demineralized water. The pigment cake was dried in an electric drying oven for 15-20 hours at C. Subsequently the pigment was finelymilled on a jet mill.

Example 2.

A calcined rutile pigment having a first post-calcination treatment wtih silica and hydrated aluminum oxide was slurried in demineralized water to form an aqueous suspension as described in Example 1 to which the following salt solutions were added for effecting the second post-calcinat-ion treatment:

(1) 31.6 ml. of a sodiumsilicate solution having a content of g.p.l. SiO corresponding to 2% SiO on the basis of pigment treated. This was followed by stirring for 10 minutes.

(2) A solution of 39.2 g. A1 (SO -1 8 H Oin 100 ml. water corresponding to 2% A1 0 on the basis of pigment treated, followed by stirring for 10 minutes.

(3) 30 ml. of a titanyl sulfate solution having a con tent of 10-0 g.p.l. TiO corresponding to 1% T10 on the basis of pigment treated; stirring was continued after the addition of the titanyl sulfate for 10 minutes.

(4) Dilute ammonia solution to pH 8.1; stirring was continued for 30 minutes and the pH readjusted to 8.1.

Subsequently the pigment was processed further as de scribed in Example 1.

Example 3 A calcined rutile pigment having a first post-calcination treatment with hydrated silica and hydrated alumina, as in Example 1, was given a second post-calcination treatment by slurrying the coated pigment in demineralized water, as described in Example 1, to. which was added the following salt solutions in sequence (1) 30 ml. of a titanyl sulfate solutionhaving a content of 100 g.p.l. corresponding to 1% TiO on the basis of pigment treated; stirring was continued for 10 minutes.

(2) 31.6 ml. of a water glass solution having a content of 190 p.g.l. SiO corresponding to 2% SiO on the basis The suspension was dey 5 of pigment treated; stirring was continued for 10 minutes after the addition. 7

(3) A solution of 39.6 g. Al (SO -18 H O in 100 ml. water corresponding to 2% A1 on the basis of pigment 6 liter of. this suspension (corresponding to 300 g. T10 was heated to 60 C. The following additions were made to the suspension consecutively while stirring constantly and maintaining the temperature substantially constant:

treated; stirring was continued for 10 minutes. (1) 15.8 ml. of a sodium silicate solution with a con- (4) Dilute ammonia solution up to pH 8.1; stirring tent of 190 g.p.l. Si0 corresponding to 1% Si0 on the was continued for 30 minutes and the pH readjusted to 8.1. basis of pigment used; after the addition of the silicate The pigment was then processed further as described in stirring was continued for minutes. Example 1, a (2) A solution of 19.6 g. Al (SO -18H O in 50 ml.

Example 4 10 water corresponding to 1% A1 0 on the basis of pig- A 81 m rutile i cut havin a first o t calcination ment treated; stirring was continued for 10 minutes.

6 c P gm g P s (3) Dilute ammonia solution up to pH 8.1; stirring treatment with hydrated silica and alumina, as m Example d f 30 t 1 was suspended in deminerahze' d water as described in was co mue or ml-nu 1 1 t h d th f H The pH was determined at several times and mamif g i e tained at 8.1 by addition, if necessary, of dilute ammonia. 2 f or c Hg 6 secon The suspension-was dewatered, washed repeatedly with ca 8 g b silicate somtion having a con demineralized water and dried in an electric drying oven at 120 C. f r1520 h fin tent of 190 g.p.l. S10 corresponding to 1% S10 on the mined 0 ou-rs The plment was then 61y bas1s of pigmtelnt tgeiaitted; stirring was continued for 10 After the first post-calcination treatment-the rutile pigmmutezafielr e a 1 ment was given a second post-calcination treatment for (2) so f of A12(S04)3'18H 2O m additional improvement in pigmentary properties as folwater correspondmg to 1% A1 0 on the bas1s of pigment lows. treatedwtmmg was cfmtmuefi for 10 mmutes: 300 g. of the treated pigment were slurried in dc 116) t ig j f ig w i P to P 5'0?smrmg was mineralized water with the addition of sodium hexaen con or mum metaphosphate as dispersin agent and of soda lye; the If A Solutlon of 9 (NHQZ 3 100 water; volume of the suspension was 1 liter, the pH 9.5-l0.0. Snmng was thfin contmued for 10 f- The suspension was then heated to 60 C. The following (5) A solution of 4.7 g.-MnSO 4H O in 50 ml. water, additions were made to the suspension while stirring corresponding to 0.5% MnO on the bas1s of pigment vigorously; treated; subsequently surnng was continued for 10 min- 1 12 1 f a Sodium Silicate solution having a utescontent of 190 g.p.l. SiO corresponding to 0.8% SiO l ammolllafiolutlon t0 P Stlmng Was on the basis of pigment treated followed by stirring for then continued for 30 minutes and pH I'&d]IlSt6d to 8.1. 10 minutes. v The pigment was then processed further as described in 2 A solution f 21 g A12(5O4)3.18H2O in 5 Example water corresponding to 1.1% A1 0 on the basis of pig- The calcined rutile pigments having two post-calcination ment treated, followed by stirring for 10 minutes. treatments according to Examples 1-4 are compared in (3) Dilute ammonia solution to a pH of 8.1, followed Table 1 below with a rutile pigment having only one calby stirring for 30 minutes. The pH was then readjusted cination treatment which was equivalent to that described 4 to 8.1. Subsequently the pigment was processed further in the first step of Example 1. as described in Example 1.

TABLE 1 Treatment of the Pigment Gloss Ex. Reten- Chalking No. tion 1 Resistance 2 First post-calcn. with Second post-calcn. with- 1.s% Slog, 2.1% 111,0, s 4 1- 1.8% $10,, 2.1% A1503--- 2.0% Slog, 2.0% A1203 27 2 2 1.8% Slog, 2.1% A1 0 2.9%, siog, 2.0% A1203, 1;0% 32 1 l a---" 1.3% SiOg, 2.1% A1 0 112%, 3 102, 2.0% Slog, 2.0% 70 01 4 1.8% 810;, 2.1%Al O 1.0%sie.,1.0% A1 0 ,0.5% 51 1 mo (as Manganese (II) Phosphate).

. 1 Gloss after 440 hrs. weathering in the Xenotest device in percent initial gloss, measured with 20 glossmeter.

2 Chalking step after 520 hrs. weathering in the Xenotest device 0=no chalking, 1=very slight cha1king, 2=slight chalking, 3=definite chalking, 4=str0ng chalking, 5=very strong Example 5 i Calcined rutile pigment was slurried in demineralized water with the addition of sodium hexametaphosphate as a. dispersing aid. The suspension was freed of all coarse particles by'wet milling in a ball mill and classifying in Example 6 A calcined rutile pigment having a first post-calcination treatment with hydrated silica and alumina, was slurried in water as described in Example 5. The following solutions were added to the suspension for the second post-calcination treatment:

(1) 30 ml. of. a titanyl sulfate solution with a content of g.p.l. TiO corresponding to 1.0% Ti0 on the basis of pigment treated; this was followed by stirring for 10 minutes.

(2) 15.8 ml. of a sodium silicate solution with a con-' a centrifuge. For the first post-calcination treatment one 75 tent of g.p.l. SiO corresponding to 1% SiO on the basis of pigment treated, followed by stirring for 10 minutes.

(3) A solution of 19.6 g. Al (SO -18H O in 50 ml. Water, corresponding to 1% A1 on the basis of are intended to be pigment treated. Stirring was thencontinued for 10 5 improved glossretention and chalking resistance which minutes. comprises: initially admixing said rutile pigment in an (4) Dilute ammonia solution up to pH 8.1, followed aqueous slurry with water-soluble salts of .aluminum and by stirring for 30 minutes. The pH was then readjusted silicon, each salt in an amount from 1.0 to 2.0% calcu-. to 8.1. Subsequently the pigment was processed further lated as the hydrous oxides on a TiO weight basis, adas in Example 1. justing the pH of the admixture to hydrolyze said salts. 7 Example 7 to precipitate and coat said pigment with. a first coating A calcined futile pigment having a first post calcina consisting of the hydrous oxides of aluminum and silition treatment with hydrated silica and alumina, was heating th.e-hydrol.ls oxlde coateq'plgment to l slurried in demineralized water as described in Example pemture sufiiclently. h1gh to sald 8 7 f 5. For the second post-calcination treatment the follow- 15 Subsequently admixing Sald oxlde coated plgment m an ing Solutions were added to the Suspension. aqueous slurry with water-soluble salts of aluminum and (1) 30 mL of a titany'l sulfate solution with a content, silicon, each salt in an amount within the range of from of 100 g.p.l. TiO corresponding to 1% Ti0 on the basis to f f as the hydrous oxldfes on a T102 of pigment .treated. This was followed by stirring for 10 Wmght adlustmg theTPH of the q to minutes drolyze said water-soluble salts and precipitate a second (2) 3L6 m1. of a Sodium silicate Solution having a coating consisting of the corresponding hydrous oxides content of 190 p.g.l. SiO corresponding to 2% SiO;-, on P sald coated Plgment, and then drymg'the coated the basis of pigment treated, followed by stirring for 10 Plgment- I minutes 2. Process according to claim 1 wherein thewater- (3) A Solution f 392 g A12(SO4)3.18H2O in 100 soluble salts used in said subsequent treatment are sodi- 1 Water corresponding to A1203 on the basis f um silicate and aluminum sulfate, each salt in an amount pigment treated, followed by stirring for 10 minutes. in the I f 0f ffoll} Q Calculated as the y- (4) Dilute ammonia solution up to pH 8.1 followed by drous OXldeS 011 a 2 Welght f stirring for minutes and readjustment of pH to 8.1. Process for 'p qmns Tutlle T 2 Pl havl ng Subsequently the pigment was processed further, as de- 30 improved g r ntwn an chalking 165151111196 Whlch scribed in Example 1. r comprises: initially admixing saidlrutile pigment in an The rutile pigments having post-calcination treatments aqueous Slurry with water'soluble salts of aluminum and according to Examples 5-7 are compared, in a second Silicon, @3611 Salt in all 3111011111; 0 9 f weatheringseries in Table II below' with a rutile pigment lated as the hydrous Oxides 011 a 2 Welght b21815, that had received only one post-calcination treatment and justing the pH of the admixture to hydrolyze said salts which had been prepared as described in the first step of toprecipitate and coat said pigment with a first coating Example 1. consisting of the hydrous oxides of aluminum and silicon,

TABLE 2 Treatment of the Pigment Gloss Ex. Reten- Chalking No. tion 1 Resistance 1 First post-calcn. with Second post-calcn. with- 1.8% S102, 2.1% A11O3- 20 4 5..." 1.0% S10 1.0% snot--- 0.8% $102, 1.1% A1 0 43 1 1.0% S10 1.0% A1203--- 1.0% Ti0,,1.o% SiO 1. a 0-1 1.0% $10,, 1.0% A1 0 1.0% 1 ib 2.0% $103, 2.0% 62 0-1 with 20 glossmeter.

2 Chalking step after 448 hours weathering in the Xenotest device.

strong chalking.

The figures recorded in Tables 1 and 2' above show clearly that it is possible by the process according to the invention to produce outstanding chalking resistance and gloss retention by a double-calcination treatment. Moreover when, in the second post-calcination treatment, a titanium salt is employed in addition to a silicate and an aluminum salt, then chalking resistance and gloss retention are even more improved. A particularly strong improvement in chalking resistance and gloss retention of the rutile pigment is obtained by adding the salts in the second post-calcination treatment in the sequence titanium salt, silicate, aluminum salt (see Table 1, Example 3).

The invention may he carried out in other specific ways than those herein set forth Without departing from the spirit and essential characteristics of the invention, and the present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equiva- 0=no chalking 1=very slight chalking, 2=sligl1t chalking, 3=definite chalking, 4=strong chalking, 5=very,

subsequently admixing said oxide coated pigment in an aqueous slurry with water-soluble salts of aluminum and silicon and one only of a water-soluble salt of a metal selected from the group consisting of titanium and manganese, each salt in an amount within the range of from 0.5 to 5.0% calculated as the hydrous oxides on a TiO weight basis, adjusting the pH of the admixture to hydrolyze said water-soluble salts and precipitate a second coating consisting of the corresponding hydrous oxides on said coated pigment, and then drying the coated pigment.

4. Process according to claim 3 wherein the hydrous oxides precipitated on said pigment by saidinitial treatment are dried prior'to subjecting said pigment to said second coating of hydrous oxides.

5. Process according to claim 3 wherein the .watersoluble salts used in said subsequent treatment are aluminum, sodium silicate, and titanyl sulfate, each of said aluminum sulfate and sodium silicate being used in an 3,251,705 9 10 amount from 1.0 to 2.0% calculated as the oxides on phate being used in an amount of about 0.5% calculated a TiO weight basis, and said titanyl sulfate being used as the oxide on a TiO weight basis. in an amount of about 1.0% calculated as the oxide on a Tioz weight basis References Cited by the Examiner 6. Process according to claim 3 wherein the water- 5 1 UNITED STATES PATENTS soluble salts used in said subsequent treatment are alumi- 704 10/1940 Erskine 300 num sulfate, sodium silicate and manganese phosphate, 2 71 3 3 954 whately 1 5 3 0 each of said aluminum sulfate and sodium silicate being 3,035 966 5/ 1962 Sim 105.400

used in an amount from 1.0% to 2.0% calculated as the oxides on a Ti0 weight basis, and said manganese phos- 10 TOBIAS E. LEVOW, Primary Examiner. 

1. PROCESS FOR PRODUCING A RUTILE TIO2 PIGMENT HAVING IMPROVED GLOSS RETENTION AND CHALKING RESISTANCE WHICH COMPRISES: INITIALLY ADMIXING SAID RUTILE PIGMENT IN AN AQUEOUS SLURRY WITH WATER-SOLUBLE SALTS OF ALUMINUM AND SILICON, EACH SALT IN AN AMOUNT FROM 1.0 TO 2.0% CALCULATED AS THE HYDROUS OXIDES ON A TIO2 WEIGHT BASIS, ADJUSTING THE PH OF THE ADMIXTURE TO HYDROLYZE SAID SALTS TO PERCIPITATE AND COAT SAID PIGMENT WITH A FIRST COATING CONSISTING OF THE HYDROUS OXIDES OF ALUMINUM AND SILICON, HEATING THE HYDROUS OXIDE COATED PIGMENT TO A TEMPERATURE SUFFICIENTLY HIGH TO DRY SAID HYDROUS OXIDES, SUBSEQUENTLY ADMIXING SAID OXIDE COATED PIGMENT IN AN AQUEOUS SLURRY WITH WATER-SOLUBLE SALTS OF ALUMINUM AND SILICON, EACH SALT IN AN AMOUNT WITHIN THE RANGE OF FROM 0.5 TO 5.0% CALCULATED AS THE HYDROUS OXIDES ON A TIO2 WEIGHT BASIS, ADJUSTING THE PH OF THE ADMIXTURE TO HYDROLYZE SAID WATER-SOLUBLE SALTS AND PRECIPITATE A SECOND COATING CONSISTING OF THE CORRESPONDING HYDROUS OXIDES ON SAID COATED PIGMENT, AND THEN DRYING THE COATED PIGMENT. 